The present invention relates to a vehicular automatic transmission for automatically shifting gears by changing power transmission paths through engagement and disengagement of gear shift means (e.g., hydraulically operated clutches).
Automatic transmissions are arranged to shift gears automatically depending on running conditions of a motor vehicle to achieve desired vehicle running characteristics. It is customary to provide a gearshift map composed of upshifting and downshifting curves for each gear position, the curves being established in relation to the vehicle speed and the engine power output, and to control the automatic transmission to shift the gears according to the gearshift map dependent on the running conditions as indicated on the gearshift map. One example of such gear shifting control is disclosed in Japanese Laid-Open Patent Publication No. 61-189354, for example.
One type of automatic transmission includes a power transmission means comprising a plurality of power transmission paths (e.g., a plurality of gear trains), a plurality of gearshift means (e.g., a plurality of hydraulically operated clutches) for selecting the power transmission paths, and a control means (e.g., a hydraulic pressure control valve) for controlling operation of the gearshift means. When a running condition of a motor vehicle, as indicated on a gearshift map, moves across an upshifting or downshifting curve, a gearshift command is produced to effect an upshift or downshift, and a solenoid valve is operated based on the gearshift command to control operation of the hydraulic pressure control valve to engage one of the hydraulically operated clutches. The power transmission path through a certain gear train associated with the engaged clutch is now selected to effect a gearshift.
The speed reduction ratio (gear ratio) of a previous gear position (i.e., a gear position provided by the power transmission path (gear train) selected until a gearshift command is issued) is different from the speed reduction ratio of a next gear position (i.e., a gear position provided by the power transmission path selected by the gearshift command). Therefore, when such a gearshift is effected, it is necessary that the automatic transmission be controlled so as not to produce a gearshift shock and a gearshift delay.
To meet the above requirement, a clutch engaging torque which will provide desired gearshift characteristics free of gearshift shocks and delays is calculated based on an engine torque to be transmitted from the engine to the hydraulically operated clutch for the next gear position, and the hydraulic pressure to be supplied to the clutch is controlled in order to produce the calculated clutch engaging torque.
It is also proposed to connect an accumulator to the hydraulically operated clutch to allow the engaging clutch torque for the next gear position to vary gradually for smooth engagement of the next-gear-position clutch, or to release the hydraulic pressure from the previous-gear-position clutch depending on a hydraulic pressure buildup in the next-gear-position clutch (see Japanese Laid-Open Patent Publication No. 60-211152, for example).
The hydraulic pressure which produces the calculated clutch engaging torque required for a gearshift has been calculated from the pushing force of the piston of the hydraulically operated clutch which will be developed by the hydraulic pressure, and the coefficient of friction of the friction plates of the hydraulically operated clutch. The hydraulically operated clutch is often mounted on the rotatable shaft of the automatic transmission. Since the hydraulically operated clutch mounted on the transmission shaft is rotated itself, it develops centrifugal forces in the oil in the hydraulic pressure chamber of the clutch because of the centrifugal forces to which the clutch is subjected. Thus, even if a constant hydraulic pressure is supplied to the hydraulically operated clutch, the hydraulic pressure in the hydraulic pressure chamber of the clutch becomes higher in a gearshift while the vehicle is running at a higher speed than in a gearshift while the vehicle is running at a lower speed, resulting in a gearshift shock during the gearshift at the higher speed.
According to one known method of suppressing gearshift shocks, the rate of change of the engine rotational speed in a gearshift is compared with a target rate of change, and the hydraulic pressure to be supplied to the hydraulically operated clutch is controlled by a feedback control loop so that the actual rate of change in the engine rotational speed will reach the target rate of change. Such method is disclosed in Japanese Laid-Open Patent Publications Nos. 60-179555, 60-151444, 60-201152, and 60-245863, for example.
Each of the gearshift means is often in the form of a friction clutch. The coefficient of friction of a friction clutch varies depending on the slip rate between the friction surfaces thereof (i.e., the relative speed between the input and output members of the clutch). Accordingly, even if the hydraulic pressure supplied to the hydraulically operated friction clutch is accurately controlled, since the coefficient of friction of the friction clutch varies, the clutch engaging forces vary, making it difficult to provide desired clutch engaging characteristics. The friction characteristics of the clutch differ depending on the material of the friction surfaces and the lubricating oil of the clutch.
FIG. 13 of the accompanying drawings illustrates one characteristic curve of the coefficient of friction by way of example. The graph of FIG. 13 shows the results of a test which was conducted using the SAE No. 2 friction testing machine. Generally, the dynamic coefficient .mu.k of friction does not vary largely as long as the slip rate is large, but has a large value immediately before the clutch is directly engaged (i.e,. in the vicinity of a time t.sub.2). Therefore, as indicated by the solid-line curve, the value of torque T for frictionally engaging the clutch becomes a sharply increasing value immediately before the time t.sub.2. The coefficient of friction immediately before the clutch is directly engaged is called a final dynamic coefficient .mu..sub.o of friction. When gearshifts are controlled using the friction clutches having characteristics as indicated by the solid-line curve of FIG. 13, even if a constant hydraulic pressure is supplied to the clutches, the clutch engaging torque rises sharply immediately before the clutch is fully engaged, and hence a gearshift shock is liable to occur.
To solve the above problem, there have been proposed such lubricating oil and friction material that the final dynamic coefficient .mu.o is smaller than the dynamic coefficient .mu.k of friction. With such an arrangement, however, since the static coefficient .mu.s of friction is small, more clutch discs have to be added, the hydraulic pressure has to be changed, and the coefficient of friction tends to vary greatly with time. For these reasons, this proposal has not yet been put to practical use.
Gearshift shocks are made smaller as the time required to effect gearshifts is longer. The gearshift time should however be selected to of an appropriate value because if the gearshift time were too long, the durability of frictional elements of the gearshift means would be adversely affected, and the driver of the vehicle would feel uneasy about the operation of the transmission. The gearshift time is equal to the time in which the input and output members of the gearshift means slip with respect to each other. The gearshift time can be of a suitable value by setting the rate of change of the ratio of the rotational speed of the input member to the rotational speed of the output member to an appropriate value.
According to the transmission control based on the rate of change of the engine rotational speed as disclosed in the above publications, a torque converter is disposed between the engine and the automatic transmission. The transmission cannot suitably be controlled because of the slippage of the torque converter. When the rotational speed and torque change greatly during gearshifts, the slippage of the torque converter varies greatly, and appropriate transmission control cannot be achieved based on the engine rotational speed.
There are known as different control modes for automatic transmissions, including a power-on/upshift mode in which the accelerator pedal is depressed and the transmission is shifted up, and a power-off/downshift mode in which the accelerator pedal is released during running of the vehicle and the transmission is shifted down as the vehicle speed is lowered. In these control modes, when the previous-gear-position gearshift means is disengaged, the difference between the rotational speeds of the input and output members of the next-gear-position gearshift means would be increased (i.e., the rotational speeds of the input and output members vary away from a synchronized speed) if the next-gear-position gearshift means remained disengaged. It is therefore necessary that the next-gear-position gearshift means start to be engaged at a proper timing.
With a hydraulically operated clutches employed as each of the gearshift means, there is a certain time lag before the gearshift means starts being operated after a gearshift command is issued. Such time lags vary depending on the different characteristics of the hydraulically operated clutches and hydraulic pressure control valves and also on the oil temperature. Thus, the timing with which the next-gear-position gearshift means start operating varies from gearshift means to gearshift means.
If the time lag becomes longer, then the engine rotational speed tends to be too high during the power-on/upshift mode, resulting in engine racing, and the engine rotational speed temporarily drops sharply during the power-off/downshift mode or the action of engine braking is reduced. The driver of the vehicle therefore feels embarrassed with respect to gearshifts. More specifically, engine racing in the power-on/upshift mode is apt to produce a gearshift shock. In the power-off/downshift mode, the driver normally expects engine braking to occur, with the result that the driver feels embarrassed by the reduction in the engine rotational speed and the reduced engine braking action. Where the vehicle is equipped with a tachometer, the engine speed indicated by the tachometer is also lowered, making the driver feel also uneasy visually with respect to the indicated engine speed.
The time lag can be shortened by increasing the hydraulic pressure to be supplied to the next-gear-position gearshift means when a gearshift command is issued. If the hydraulic pressure were too high, the next-gear-position gearshift means would be engaged too abruptly, producing a gearshift shock.